This invention relates to non-carbon, metal-based anodes provided with an electrochemical active surface coating for use in cells for the electrowinning of aluminium by the electrolysis of alumina dissolved in a molten fluoride-containing electrolyte, and to methods for their fabrication and reconditioning, as well as to electrowinning cells containing such anodes and their use to produce aluminium.
The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite, at temperatures around 950xc2x0 C. is more than one hundred years old.
This process, conceived almost simultaneously by Hall and Hxc3xa9roult, has not evolved as many other electrochemical processes.
The anodes are still made of carbonaceous material and must be replaced every few weeks. The operating temperature is still not less than 950xc2x0 C. in order to have a sufficiently high solubility and rate of dissolution of alumina and high electrical conductivity of the bath.
The carbon anodes have a very short life because during electrolysis the oxygen which should evolve on the anode surface combines with the carbon to form polluting CO2 and small amounts of CO and fluorine-containing dangerous gases. The actual consumption of the anode is as much as 450 Kg/Ton of aluminium produced which is more than ⅓ higher than the theoretical amount of 333 Kg/Ton.
In the second largest electrochemical industry following aluminium, namely the chlorine and caustic industry, the invention of dimensionally stable anodes (DSA(copyright)) which were developed around 1970 permitted a revolutionary progress in chlorine cell technology resulting in a substantial increase in cell energy efficiency, in cell life and in chlorine caustic purity.
The substitution of graphite anodes with DSA(copyright) increased drastically the life of the anodes and reduced substantially the cost of chlorine production and operating the cells.
In the case of aluminium production, an additional problem is the pollution due to the materials used in the process and to the primitive cell design and operation which have remained the same over the years.
Progress has been made in the operation of modern plants which utilise cells where the gases emanating from the cells are in large part collected and adequately scrubbed and where the emission of highly polluting gases during the manufacture of the carbon anodes and cathodes is carefully controlled.
However, the frequent substitution of the anodes in the cells is still a clumsy and unpleasant operation. This cannot be avoided or greatly improved due to the size and weight of the anode and the high temperature of operation.
Thus, the dimensionally-stable anode technology used in chlorine production has not yet been successfully adapted to the aluminium electrowinning cells.
Several improvements were made in order to increase the lifetime of the anodes of aluminium electrowinning cells, usually by improving their resistance to chemical attacks by the cell environment and air to those parts of the anodes which remain outside the bath. However, most attempts to increase the chemical resistance of anodes were coupled with a degradation of their electrical conductivity.
U.S. Pat. No. 4,614,569 (Duruz et al.) describes anodes for aluminium electrowinning coated with a protective coating of cerium oxyfluoride, formed in-situ in the cell or pre-applied, this coating being maintained by the addition of cerium to the molten cryolite electrolyte. This made it possible to have a protection of the surface only from the electrolyte attack and to a certain extent from the gaseous oxygen but not from the nascent monoatomic oxygen.
EP Patent application 0 306 100 (Nyguen/Lazouni/Doan) describes anodes composed of a chromium, nickel, cobalt and/or iron based substrate covered with an oxygen barrier layer and a ceramic coating of nickel, copper and/or manganese oxide which may be further covered with an in-situ formed protective cerium oxyfluoride layer.
Likewise, U.S. Pat. Nos. 5,069,771, 4,960,494 and 4,956,068 (all Nyguen/Lazouni/Doan) disclose aluminium production anodes with an oxidised copper-nickel surface on an alloy substrate with a protective barrier layer. However, full protection of the alloy substrate was difficult to achieve.
A significant improvement described in U.S. Pat. No. 5,510,008, and in International Application WO96/12833 (Sekhar/Liu/Duruz) involved micropyretically producing a body from nickel, aluminium, iron and copper and oxidising the surface before use or in-situ. By said micropyretic methods materials have been obtained whose surfaces, when oxidised, are active for the anodic reaction and whose metallic interior has low electrical resistivity to carry a current from high electrical resistant surface to the busbars. However it would be useful, if it were possible, to simplify the manufacturing process of these materials and increase their life to make their use economic.
Metal or metal-based anodes are highly desirable in aluminium electrowinning cells instead of carbon-based anodes. As described hereabove, many attempts were made to use metallic anodes for aluminium production, however they were never adopted by the aluminium industry because of their poor performance.
An object of the invention is to reduce substantially the consumption of an applied electrochemically active anode surface coating of a metal-based non-carbon anode for aluminium electrowinning cells which coating is in contact with the electrolyte.
Another object of the invention is to provide a surface coating for a metal-based anode for aluminium electrowinning cells which in addition to a long life has a high electrochemical activity and can easily be applied onto an anode substrate.
A major object of the invention is to provide an anode for the electrowinning of aluminium which has no carbon so as to eliminate carbon-generated pollution and reduce the high cell operating costs.
The invention relates to a non-carbon, metal-based, high temperature resistant, electrically conductive and electrochemically active anode of a cell for the production of aluminium by the electrolysis of alumina dissolved in a fluoride-containing electrolyte. The anode has a metal-based substrate to which an adherent surface coating is applied prior to its immersion into the electrolyte and start up of the electrolysis by connection to the positive current supply. The coating is obtainable from one or more layers applied from: a liquid solution, a dispersion in a liquid or a paste, a suspension in a liquid or a paste, and a pasty or non-pasty slurry, and combinations thereof with or without one or more further applied layers, with or without heat treatment between two consecutively applied layers when at least two layers are applied. The coating is after final heat treatment electrically conductive and during operation in the cell electrochemically active for the oxidation of oxygen ions present at the surface of the anode to form monoatomic nascent oxygen which as such or as biatomic molecular gaseous oxygen oxidises or further oxidises the surface of the coating, or part or most of the coating or the surface of the substrate, to form a barrier to the ionic and gaseous oxygen and even a barrier to the nascent monoatomic oxygen, the coating having a slow dissolution rate in the fluoride-containing electrolyte.
In the context of this invention:
a metal-based anode means that the anode contains at least one metal in the anode substrate as such or as alloys, intermetallics and/or cermets.
a liquid solution means a liquid containing ionic species which are smaller than 5 nanometers and/or polymeric species of 5 to 10 nanometers and no larger particles;
a dispersion means a liquid containing particles in colloidal form, wherein the size of the largest particles is comprised between 10 and 100 nanometers;
a suspension means a liquid containing particles in which the largest particles are comprised between 100 and 1000 nanometers; and
a slurry means a liquid containing particles the size of which exceeds 1000 nanometers.
The metal-based substrate is usually selected from a metal, an alloy, an intermetallic compound or a cermet. The substrate may possibly comprise stoichiometric or sub-stoichiometric compounds, in particular oxides.
Advantageously, the metal-based substrate comprises at least one metal selected from nickel, copper, cobalt, chromium, molybdenum, tantalum and iron, as a metal and/or as an oxide. For instance, the metal substrate is an alloy consisting of 10 to 30 weight % of chromium, 55 to 90% of at least one of nickel, cobalt or iron, and up to 15% of aluminium, titanium, zirconium, yttrium, hafnium or niobium.
Preferably, the metal-based substrate comprises a surface pre-coating or pre-impregnation. The pre-coating or pre-impregnation may for instance comprise ceria.
The applied layer may comprise one or more oxides, oxyfluorides, phosphides, carbides and combinations thereof such spinels, and/or perovskites. For instance. The electrochemically active layer may contain doped, non-stoichiometric and/or partially substituted spinels, the doped spinels comprising dopants selected from the group consisting Ti4+, Zr4+, Sn4+, Fe4+, Hf4+, Mn4+, Fe3+, Ni3+, Co3+, Mn3+, Al3+, Cr3+, Fe2+, Ni2+, Co2+, Mg2+, Mn2+, Cu2+, Zn2+ and Li+.
The oxide may be present in the electrochemically active layer as such, or in a multi-compound mixed oxide and/or in a solid solution of oxides. The oxide may be in the form of a simple, double and/or multiple oxide, and/or in the form of a stoichiometric or non-stoichiometric oxide.
The applied layer may comprise a ferrite, such as a ferrite selected from cobalt, manganese, nickel, magnesium and zinc ferrite, and mixtures thereof. The ferrite may be doped with at least one oxide selected from chromium, titanium, tin and zirconium oxide. Nickel ferrite may be partially substituted with Fe2+.
Alternatively, the applied layer may comprise a chromite, such as a chromite selected from iron, cobalt, copper, manganese, beryllium, calcium, strontium, barium, magnesium, nickel and zinc chromite.
Advantageously, the applied layer may comprise an electrocatalyst for the formation of molecular oxygen from atomic oxygen, selected from iridium, palladium, platinum, rhodium, ruthenium, silicon, tin and zinc, the Lanthanide series and Mischmetal, and their oxides, mixtures and compounds thereof.
The layer may also comprise one or more dried colloids or polymers selected from the group consisting of colloidal alumina, silica, yttria, ceria, thoria, zirconia, magnesia, lithia, tin oxide, zinc oxide, monoaluminium phosphate or cerium acetate. The colloid or polymer may be derived from colloid or polymer precursors and reagents which are solutions of at least one salt such as chlorides, sulfates, nitrates, chlorates, perchlorates or metal organic compounds such as alkoxides, formates, acetates of aluminium, silicon, yttrium, cerium, thorium zirconium, magnesium and lithium. Possibly, the solutions of metal organic compounds, principally metal alkoxides, are of the general formula M(OR)z where M is a metal or complex cation, R is an alkyl chain and z is a number, preferably from 1 to 12. The colloid or polymer precursor or reagent may also contain a chelating agent such as acetyl acetone or ethylacetoacetate.
The invention also relates to a method of manufacturing such an anode. The method comprises forming onto a metal-based substrate one or more layers applied from: a liquid solution, a dispersion in a liquid or a paste, a suspension in a liquid or a paste, and a pasty or non-pasty slurry, and combinations thereof with or without one or more further applied layers, with or without heat treatment between two consecutively applied layers when at least two layers are applied. The coating is then exposed to a final heat treatment so as to render it electrically conductive and electrochemically active during operation in the cell.
Several techniques may be used to apply the layers such as painting, spraying, dipping, brush, electrodeposition or rollers.
A solution, dispersion, suspension or slurry may also be applied in a very liquid, a liquid, a thick or pasty form.
When several liquid-containing layers are applied, each layer may be allowed to dry at least partially in the ambient air or assisted by heating before applying the next layer.
The coating may be also formed by applying onto the metal-based substrate a precursor containing constituents which react among themselves to form the coating, and reacting the constituents to form the coating. Alternatively, the coating may be formed by applying onto the metal-based substrate a precursor containing at least one constituent which reacts with the metal-substrate to form the coating, and reacting the constituent(s) with the metal-substrate to form the coating.
A solid-applied layer may be applied onto the metal-substrate by plasma spraying, physical vapour deposition, chemical vapour deposition or calendering rollers.
The above methods may also be applied for reconditioning an anode as described above whose electrochemically active layer is worn or damaged. The method comprises clearing at least worn and/or damaged parts of the active coating from the substrate and then reconstituting at least the electrochemically active coating.
A further object of the invention is a cell for the production of aluminium by the electrolysis of alumina dissolved in a molten fluoride-containing electrolyte, such as cryolite, comprising one or more anodes as described hereabove.
Preferably, the cell comprises at least one aluminium-wettable cathode. Even more preferably, the cell is in a drained configuration by having at least one drained cathode on which aluminium is produced and from which aluminium continuously drains.
The cell may be of monopolar, multi-monopolar or bipolar configuration. A bipolar cell may comprise the anodes as described above as a terminal anode or as the anode part of a bipolar electrode.
Advantageously, the cell may comprise means to circulate the electrolyte between the anodes and facing cathodes and/or means to facilitate dissolution of alumina in the electrolyte.
The cell may be operated with the electrolyte at conventional temperatures, such as 950 to 970xc2x0 C., or at reduced temperatures as low as 700xc2x0 C.
Another object of the invention is a method of producing aluminium in a such a cell, comprising dissolving alumina in said fluoride-containing electrolyte and then electrolysing the dissolved alumina to produce aluminium.